Cascode amplifiers are known in the art for converting current to voltage. Current to voltage conversion is particularly useful when a comparison between a first current and a second current is required. The reason is that voltage comparators, such as operational amplifiers, for example, are readily available for comparing two voltage values. Accordingly, the conventional approach in comparing two current values involves first converting the current values to voltage values, and then comparing the voltage values using an operational amplifier.
In practice, the comparison of current values is useful in a wide variety of applications. For example, often the state of a device or component is indicated by the current associated with the device or component. In the case of a memory device, for example, the state of a memory cell within the memory device is typically indicated by the current drawn by the memory cell. For example, a memory cell may be defined as a “programmed” cell if the memory cell current is below a reference current value. Conversely, a memory cell may be defined as an “erased” cell if the memory cell current is above the reference current value. In this example, a comparison between the detected memory cell current and the reference current is needed to determine the state of the memory cell. As pointed out above, in practice, the memory cell current and the reference current are first converted to corresponding voltage values, and then the converted voltage values are compared using an operational amplifier.
Known cascode amplifiers suffer from several problems. First, while it is desirable to stabilize the voltage at the node connecting the cascode amplifier to the memory cell (i.e., the bit line voltage), it is often difficult to do so. The reason is that due to variations, such as variations in supply voltage, process and temperature, the threshold voltage (VT) of the transistors implemented in the cascode amplifier may have a wide varying range. Since the transistors implemented in the cascode amplifier are typically of different types (e.g., have different threshold voltage ranges), the transistors do not closely track each other with respect to these variations, thereby resulting in a bit line voltage which varies greatly and depends largely on such variations. An unstable bit line voltage may lead to an unreliable output voltage from the cascode amplifier. Accordingly, there exists a strong need in the art to overcome deficiencies of known cascode amplifier circuits, such as those described above, and to provide fast, stable and accurate bit line voltages.